The present invention relates to a color ink jet printer for printing color images on a transported recording medium, and more specifically, to a color ink jet printer which is provided with a plurality of ink jet units corresponding to plural colors of ink including black, each one of which is provided with a plurality of ink jet nozzles arranged along a direction intersecting a transportation direction of the recording medium, and can record color images by ejecting plural colors of ink including black in the direction intersecting the transportation direction of the recording medium onto the transported recording medium.
This kind of color ink jet printer has recently begun to be used in place of a conventional, so-called serial type color ink jet printer which can record color images by ejecting plural colors of ink onto a transported recording medium while the ink jet unit provided with a plurality of ink cassettes for plural colors of ink is making a reciprocal movement along the recording medium in a direction intersecting a transportation direction of the recording medium (in a width direction of the recording medium).
This new type of conventional color ink jet printer is widely known by, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-138520, and in comparison with the conventional so-called serial type color ink jet printer, it has following advantages. That is, since it is no longer necessary to reciprocate the ink jet unit to record color images, it can record color images on the recording medium at high speed. In addition, since the size of each of a plurality of ink cassettes combined with the non-reciprocating ink jet unit can be increased, the frequency for replacing ink cassettes is reduced (that is, recording efficiency is high).
Next, referring to FIG. 8, a structure of the new type of conventional color ink jet printer will be schematically explained.
As shown in FIG. 8, the new type of conventional color ink jet printer 10 has a paper sheet cassette 14 removably arranged at a lower part of an inner space of an external housing 12. The paper sheet cassette 14 includes a mounting plate 14a on which a large number of paper sheets P are stacked and an urging means 14b such as a spring which urges the mounting plate 14a upwards. Substantially at a center in the inner space, a rotation drum 16 which rotates at a predetermined speed in a direction indicated by an arrow R is arranged.
From one end of the paper sheet cassette 14, a first transportation unit 18 extends towards a side facing portion of an outer circumferential surface of the rotation drum 16. The first transportation unit 18 includes a pick-up roller 18a with which an upper most paper sheet P in the large number of paper sheets urged upwards as described above by the urging means 14a in the paper sheet cassette 14 comes in contact and which picks up the uppermost paper sheet P by its rotation, and a resist roller pair 18b with which a leading end of the paper sheet P picked up by the pick-up roller 18a from the cassette 14 comes in contact.
The first transportation unit 18 is connected at the resist roller pair 18b with a branched transportation unit 20 extending from the resist roller pair 18b to a paper sheet manual supply opening 10a formed at a predetermined position of an outer wall of the external housing 12. The paper sheet manual supply opening 10a is usually covered with a paper sheet manual supply lid 22 which is mounted in a predetermined position on the outer wall of the external housing 12 to be rotatable between an open position where the lid 22 is left from the opening 10a to open the opening 10a and a closed position where the lid 22 covers the opening 10a to close the opening 10a. In FIG. 1, the paper sheet manual supply lid 22 is arranged in the open position.
The paper sheet manually inserted into the opened paper sheet manual supply opening 10a is transported to the resist roller pair 18b by a sub pick-up roller 20 arranged close to the paper sheet manual supply opening 10a in the inner space of the external housing 12.
At a predetermined position along each of the transportation paths of the first transportation unit 18 and the branched transportation unit 20, a transportation fault (jamming) sensing unit 24 which senses a transportation fault (jamming) of the paper sheet P in the transportation path is arranged.
The rotation drum 16 includes a recording medium holding member (not-illustrated) mounted in a predetermined position on the outer circumferential surface of the drum 16 so as to be movable between a closed position where the holding member is laid on the circumferential surface and an opening position where the holding member is left from the circumferential surface. The holding member (not-illustrated) is opened and closed by a holding member driving unit (not-illustrated)located in the vicinity of the rotation drum 16. The holding member (not-illustrated) is opened by the holding member driving unit (not-illustrated) when it reaches a predetermined rotation angle position as the rotation drum 16 makes the predetermined rotation.
The resist roller pair 18b of the first transportation unit 18 supplies the paper sheet towards the opened holding member on the rotation drum 16. The opened holding member is closed by the holding member driving unit (not-illustrated) at a timing when the leading end of the paper sheet reaches a gap between the opened holding member and the outer circumferential surface of the rotation drum 16, so that the leading end of the paper sheet is held at the predetermined position on the outer circumferential surface of the rotation drum 16 by the closed holding member.
Thereafter, as the predetermined rotation of the rotation drum 16 takes place, the paper sheet is pressed against and brought in close contact with the outer circumferential surface of the rotation drum 16 by a pressure roller 16a and an electric charging roller 16b arranged on the circumferential surface of the rotation drum 16. The pressure roller 16a and the charging roller 16b are separated from the circumferential surface of the rotation drum 16 after the paper sheet is passed under these rollers 16a and 16b. At a predetermined position adjacent to the outer circumferential surface of the rotation drum 16, a transportation fault (jamming) sensing unit 16c which senses a separation of the paper sheet from the outer circumferential surface of the rotation drum 16 (that is, transportation fault and a cause of jamming) is arranged.
In this embodiment, the rotation drum 16, pressure roller 16a, charging roller 16b, and the not-illustrated holding member and the holding member driving unit mentioned above form a recording medium holding unit which holds and rotates the paper sheet as the recording medium.
In the inner space of the external housing 12, a recording head 26 is further arranged adjacent to the outer circumferential surface of the rotation drum 16. The recording head 26 is supported by a recording head moving unit 28, and the recording head moving unit 28 can move the recording head 26 between a recording position where the recording head 26 is located closely to the outer circumferential surface of the rotation drum 16 as shown by a solid line in FIG. 1 and a retracted position where the recording head 26 is brought away from the outer circumferential surface of the rotation drum 16 as indicated by a two-dots chain line in FIG. 1. In the recording position, the recording head 26 is close to the outer circumferential surface of the rotation drum 16 by 1 mm or less.
The recording head 26 includes four ink jet units 26C, 26M, 26Y, and 26K arranged at four positions separated from each other with a predetermined distance interposed therebetween along the circumferential direction of the outer circumferential surface of the rotation drum 16. Each of the four ink jet units 26C, 26M, 26Y, and 26K extends in a range substantially corresponding to a width of the rotation drum 16 in a direction along a rotation center shaft 16d of the rotation drum 16.
FIG. 9 shows an enlarged view of a portion of the recording head 26, the portion facing the outer circumferential surface of the rotation drum 16. As shown in FIG. 9, each of the four ink jet units 26C, 26M, 26Y, and 26K is constructed by combining independent sections 27 one another in the direction along the rotation center shaft 16d. In this combination, each of the independent sections 27 is elongated and they are generally arranged along a straight line with their longitudinal directions being located along the direction along the rotation center shaft 16d, but in detail, they are staggered alternately along the straight line.
Each of the independent sections 27 has a large number of ink jet nozzles 27a arranged at a predetermined pitch in a portion facing to the outer circumferential surface of the rotation drum 16, and the ink jet nozzles 27a of all of the sections of each ink jet unit 26C, 26M, 26Y or 26K are arranged in a range substantially corresponding to a width of the rotation drum 16 in the direction along the rotation center shaft 16d. In this conventional example, the predetermined pitch is set in such a manner that the ink jet nozzles 27a are arranged at 300 dpi (dots per inch). And the predetermined pitch of a large number of ink jet nozzles 27a in each of the four ink jet units 26C, 26M, 26Y, and 26K is the same one another.
It is possible to prepare each of the ink jet units 26C, 26M, 26Y, and 26K with a large number of ink jet nozzles 27a easily and at low cost by forming each of the ink jet units 26C, 26M, 26Y, and 26K with a plurality of independent sections 27 and by forming a large number of ink jet nozzles 27a in each of a plurality of independent sections 27. This is because the more the length of a member in which a large member of microscopically small ink jet nozzles 27a must be formed is large, the more a process for forming a large number of microscopically small ink jet nozzles 27a at a predetermined pitch over the length of the member complicate.
The four ink jet units 26C, 26M, 26Y, and 26K are connected to four ink containers 30 provided in the inner space of the external housing 12 through ink supply tubes 32 with ink supply pumps 31 each having a valve function. To the four ink containers 30, ink cassettes 34 containing ink of colors (in this embodiment, black, cyan, magenta, and yellow) corresponding to ink which are used in the ink jet units 26C, 26M, 26Y, and 26K, are removably provided. The recording head 26 can reciprocate in the direction along the rotation center shaft 16a of the rotation drum 16 by 1/2 the pitch between a large number of ink jet nozzles 27a of each of the four ink jet units 26C, 26M, 26Y, and 26K.
The ink supply pumps 31 open the ink supply tubes 32 when the four ink jet units are used to form color images, and allows the ink to be automatically supplied by a negative pressure generated in the four ink jet units as a result of ejection of ink from the ink jet nozzles 27a, from four ink containers 30 to four ink jet units 26C, 26M, 26Y, and 26K. In this case, the ink jet unit 26c ejects the cyan ink, the ink jet unit 26M ejects the magenta ink, the ink jet unit 26Y ejects the yellow ink, and the ink jet unit 26K ejects the black ink.
The ink supply pump 31s are used to prevent the ink jet nozzles 27a from clogging by forcibly discharging out the ink from the ink jet nozzles 27a of the four ink jet units 26C, 26M, 26Y, and 26K.
In the color ink jet printer 10 of this embodiment, the number of rotation of the rotation drum 16 which is necessary to hold the paper sheet supplied to the rotation drum 16 onto the predetermined position on the outer circumferential surface of the rotation drum 16 is 1. And the number of rotation of the rotation drum 16 which is necessary to form a full-color image on the paper sheet held on the rotation drum 16 at the 300 dpi (dot per inch) pixel density (dissolution) based on an image signal sent from electrical control circuit (not illustrated) is 2. In the similar manner, the number of rotation of the rotation drum 16 which is required for forming a full-color image on the paper sheet held on the rotation drum 16 at the 600 dpi (high dissolution) is 4. And the paper sheet on which the full-color image have been formed is separated from the rotation drum 16 while the rotation drum 16 is making further one rotation, and is transported to an outside of the color ink jet printer 10 by a second transport unit later described.
When the recording head 26 forms the full-color image, the recording head 16 forms one half of the full-color image in the direction along the rotation center shaft 16a of the rotation drum 16 white the rotation drum 16 makes one rotation. Thereafter, the recording head 26 moves in the direction along the rotation center shaft 16a by 1/2 pitch, and forms the remainder one half of the full-color image in the direction along the rotation center shaft 16a of the rotation drum 16 white the rotation drum 16 makes the next one rotation.
That is, when the recording head 26 forms the full-color image on the paper sheet at the 300 dpi pixel density (resolution) while the rotation drum 16 makes two rotations, the recording head 26 makes one reciprocal movement in the direction along the rotation center shaft 16a by 1/2 pitch. When the recording head 26 forms the full-color image on the paper sheet at the high pixel density (high resolution) of 600 dpi while the rotation drum 16 makes 4 rotations, the recording head 26 makes two reciprocal movements in the direction along the rotation center shaft 16a.
Consequently, this conventional color ink jet printer 10 requires 4 rotations of the rotation drum 16 to hold the paper sheet on the outer circumferential surface of the rotation drum 16, form the full-color image on the paper sheet at the 300 dpi pixel density, and separate the paper sheet on which the full-color image have been formed from the outer circumferential surface of the rotation drum 16, and in the similar manner, 6 rotations of rotation drum 16 are required for forming the full-color image on one paper sheet at the pixel density of 600 dpi (high resolution).
Assuming the rotation speed of the rotation drum 16 in this case be 120 rpm, this conventional color ink jet printer 10 can form the full-color image of 600 dpi high pixel density (high resolution) at 20 sheets per minute, and in the case of 300 dpi pixel density full-color image, it can form the full-color image on 30 sheets per minute.
The recording head 26 includes an ink catcher 36 which is movable between a retracted position where the ink catcher 26 is left from the portion of the recording head 26 facing the outer circumferential surface of the rotation drum 16 and a covering position where the ink catcher 36 covers the portion of the recording head 26. The ink catcher 36 is arranged in the retracted position while the recording head 26 is located in the recording position as shown by the solid line in FIG. 1, and is arranged in the covering position while the recording head 26 is located at in retracted position as shown by the two-dots chain line in FIG. 1. The ink catcher 36 in the covering position prevents the ink leaking out from the ink jet nozzles 27a from soiling the outer circumferential surface of the rotation drum 16. The ink supply pumps 31 operate to prevent the ink jet nozzles 27a of the four ink jet units 26C, 26M, 26Y, and 26K from clogging while the ink catcher 36 is arranged in the covering position.
In the ink catcher 36, an ink jet nozzle cleaning device (not-illustrated) is assembled. This ink jet nozzle cleaning device is used while the ink catcher 36 is arranged in the closed position to forcibly and mechanically clean the ink jet nozzles 27a after the recording head 26 is used over a long period of time or the recording head 26 is not used over a long period of time.
In the inner space of the external housing 12, the second transportation unit 38 is arranged to extend from a portion in the downstream side of the recording head 26 in the predetermined rotation direction R of the rotation drum 16 in the vicinity of the outer circumferential surface of the rotation drum 16, to the upper wall of the external housing 12. The second transportation unit 38 includes a peeling member (not illustrated) at its base end close to the outer circumferential surface of the rotation drum 16. The peeling member is used for peeling the paper sheet on which the color image have been formed from the outer circumferential surface of the rotation drum 16 and for introducing the peeled paper sheet into the second transportation unit 38.
The holding member (not illustrated) of the rotation drum 16 is opened by the holding member driving unit (not illustrated) at a predetermined rotation angle position immediately before holding member reaches at the base end of the second transportation unit 38 in last one rotation of the rotation drum 16 after the color image is formed on the paper sheet on the outer circumferential surface of the rotation drum 16 as described above by the recording head 26. As a result of this, the peeling member (not illustrated) at the base end of the second transportation unit 38 peels the paper sheet on which the color image have been formed, from the outer circumferential surface of the rotation drum 16 and introduces into the second transportation unit 38.
The second transportation unit 38 includes a plurality of paper sheet transportation roller pairs 38a and transportation fault (jamming) sensing units 38b at a plurality of positions along the transportation path of the second transportation unit 38.
In this embodiment, the upper wall 12a of the external housing 12 can move horizontally between a closed position in which the upper wall covers the upper opening of the external housing 12 as shown in FIG. 1 and an open position in which the upper wall opens a part of the upper opening. While the upper wall 12a is arranged in the closed position as shown in FIG. 1, the paper sheet on which the color image have been formed is discharged from an extending end of the second transportation unit 38 onto the upper surface of the upper wall 12a in the closed position as indicated by an arrow A.
When the upper wall 12a is located in the open position, it is possible to replace four ink cassettes 34 through the part of the upper opening of the external housing 12.
In the conventional color ink jet printer 10 structured as described above, the color of the ink most frequently used is black, and it is frequent that the black ink only is used. This is because characters and figures are most frequently formed.
And in forming images by the black ink which is most frequently used, it is strongly desired to improve the image forming quality (that is, pixel density) without degrading the image forming speed (that is, the speed for forming the image per unit time), or to improve the image forming speed (that is, the speed to form the image per unit time) without degrading the image forming quality (that is, pixel density).
Under the above-mentioned circumstances, it is an object of the present invention to provide a color ink jet printer which can improve the image forming quality (that is, pixel density) without degrading the image forming speed (that is, the speed for forming the image per unit time) or to increase the image forming speed (that is, the speed for forming the image per unit time) without degrading the image forming quality (that is, pixel density) in forming the image with only the inks most frequently used, such as black ink only.